This is often caused by using a special character, are you using any special characters in your instructable?

I've got the same error in one step of my newest Instructable: http://www.instructables.com/id/Mayan-Rubiks-Treasure-Box/#step1
I get it every time I try to edit the step.

I've got the newest JAVA version, tried it in the latest firefox and IE versions as well as in ubuntu and windows... With no luck.

a JSON error is due to JAVA not loading on your browser correctly. Make sure you have the latest version of JAVA for your browser, along with no script blocking add-ons (like Ad block or any of the other browser add ons I can see in your screen shot).

Hey mygms,

I think there was probably just something gumming up the works. If you try again (to make the instructable from scratch) I think everything should work fine. I went ahead and deleted that one for you.

I saved the text from the instructable in case you needed it, if you do, please pm me and I'll send it to you straight away.

Please let me know if you have any more troubles!

-Stump

Kudos on the screen-shot.

Bug report filed.